Technical Field
The present disclosure relates to a system for driving an array of MEMS (Micro-ElectroMechanical System) structures and to a corresponding driving method.
Description of the Related Art
It is known that a MEMS structure in general comprises at least one mobile element (for example a membrane, or diaphragm) capacitively coupled to at least one fixed element.
Application of a potential difference between said mobile and fixed elements generates a movement of the mobile element with respect to the fixed element, enabled by an appropriate arrangement of elastic elements; in particular, in the case of a speaker, the movement is such as to generate a sound wave.
Likewise, an external stress, such as to move the mobile element with respect to the fixed element, generates a potential difference between the same fixed and mobile elements which may be detected to obtain an indication of the value of the external stress.
From the circuit standpoint, a MEMS structure may thus be represented by at least one capacitor defining a capacitance between two terminals, one connected to the mobile element and the other connected to the fixed element.
Recently, applications have been proposed that envisage the use of arrays of MEMS structures (in brief, MEMS arrays), organized in rows and columns; for example, in the audio field, realization of a speaker device has been proposed, constituted by an array of MEMS structures.
In this case, each MEMS structure defines an elementary unit, also known as pixel, comprising a membrane, which is able to move towards one or more fixed armatures; application of an appropriate potential difference between the membrane and the fixed armatures enables movement of the membrane itself into contact with the armatures, with consequent generation of elementary sound waves. The superposition of the elementary sound waves generated by the MEMS structures of the array generates the resulting sound signal produced by the MEMS speaker.
The manufacturing of MEMS structures arranged in an array, in some applications use free access to an external environment (for example, audio applications for reception or emission of sound waves) and may envisage the absence of surface passivation; in other words, the resulting MEMS array may be directly exposed to air, without the presence of protection structures.
It follows that the MEMS array is more readily subject to the penetration of contaminating particles or entities, which may create failure, for example consisting of undesired current paths between the terminals of one or more of the MEMS structures of the array, rendering them no longer functional or reliable and thus unusable.
In general, the risk may arise of short-circuits which may activate a high current path, with the danger of damage to the MEMS structures (for example, to the corresponding metallizations) and to the driving systems electrically coupled to the MEMS array.
By way of example, FIG. 1 is a schematic illustration of a MEMS array 1, including a plurality of MEMS structures 2, set in rows and columns; each MEMS structure 2 is identified by the respective number of row and column to which it is associated: for example, the MEMS structure ‘21’ is associated to the second row and to the first column.
In particular, each MEMS structure 2, in the example represented, has a first terminal 2a, for example associated to a corresponding membrane (not illustrated herein) connected to a respective column and a second terminal 2b, for example associated to a corresponding fixed armature (not illustrated herein) connected to a respective row; the MEMS structure 2 defines a capacitor between the first and second terminals 2a, 2b. 
It will be evident, however, for a person skilled in the field, that the MEMS structure 2 may differ from what has been illustrated schematically, for example in that it presents one or more further terminals, in this case coupled to respective rows and/or columns. It is further evident that, alternatively, the first terminal 2a of the MEMS structure 2 may be connected to a respective row and the second terminal 2b to a respective column of the MEMS array 1.
For each row i (where i is an integer ranging from 1 to n, n being the total number of rows), the MEMS array 1 includes a row driving stage 4, having an input 4a, which receives (for example, from a control unit of an external device) a respective row-address signal Ri and an output 4b, which supplies a corresponding row-biasing signal DRi, having an appropriate value that is a function of the row-address signal Ri. The output 4b is connected to the second terminal 2b of the MEMS structures 2 connected to the respective row.
In particular, the row driving stages 4 are equal in number to the rows of the MEMS array 1 and are thus configured so as to bias at one or more appropriate biasing voltages, according to the possible operating conditions, the second terminal 2b of all the MEMS structures 2 connected to the respective row.
Likewise, for each column j (where j is an integer ranging from 1 to m, m being the total number of columns, which may possibly be equal to the number of rows), the MEMS array 1 includes a column driving stage 6, having an input 6a, which receives a respective column-address signal Cj and an output 6b, which supplies a corresponding column-biasing signal DCj, having an appropriate value that is a function of the column-address signal Cj. The output 6b is connected to the first terminal 2a of the MEMS structures 2 connected to the respective column.
In particular, the column driving stages 6 are equal in number to the columns of the MEMS array 1 and are thus configured so as to bias to one or more appropriate biasing voltages, according to the possible operating conditions, the first terminal 2a of all the MEMS structures 2 connected to the respective column.
Consequently, the aforesaid row-address and column-address signals Ri, Cj define potential differences of a desired value between the first and second terminals 2a, 2b of the MEMS structures 2 of the MEMS array 1.
The row and column driving stages 4, 6 are designed so as to take into account the total of the capacitive loads defined on the respective row and/or column by the associated MEMS structures 2 and further possible intrinsic resistances in the paths required for reaching the same MEMS structures 2.
The present Applicant has realized that the architecture described for the MEMS array 1 has some drawbacks.
In particular, in the case where a failure arises, for example owing to onset of a short-circuit path due to infiltration of impurities, regarding even just one of the MEMS structures 2, the entire MEMS array 1 may prove unusable, or in any case no longer able to operate as expected.
Evidently, this drawback, in addition to representing a problem from the standpoint of the operating costs, may not be accepted in given applications in which a certain level of tolerance to failures is required.